Tyre sizes and what they mean   185-65 HR13 185 This is the width in mm of the tyre from sidewall to sidewall when it's unstressed and you're looking at it head on (or top-down) 65 This is the height of the tyre sidewall, or section height, expressed as a percentage of the width. It is known as the aspect ratio. In this case, 65% H This is the speed rating of the tyre (click here) R This tells you that the tyre is a radial construction 13 This is the diameter in inches of the rim of the wheel that the tyre has been designed to fit on. Don't ask why tyre sizes mix imperial and metric measurements. They just do.